This invention relates to a portable or hand-held apparatus having a battery as a main power source, and more particularly, to a circuit for an apparatus which consumes a varying current during operation, for checking the battery voltage of the apparatus, irrespective of the load variation of the apparatus.
Portable and hand-held apparatuses are being used in increasing numbers. Most apparatuses of this type are provided with a batteries for main power sources. In some types of personal computers and word processors, the batteries are used to drive mechanical units such as a floppy disk drive (FDD) and a printer. As is known, these mechanical units consume tens to hundreds of times more current than the electric components of the apparatuses. The power remaining in the battery is often insufficient for normal operation of the mechanical units after a long use of the electric components. Hence, the remaining power must be and actually is detected before the mechanical unit are driven.
FIG. 8 shows a conventional battery-voltage check circuit incorporated in a battery-driven, portable or hand-held apparatus. This circuit detects the remaining power of a battery, in terms of the voltage between the terminals of the battery. When this voltage (hereinafter referred to as "output voltage") falls below a reference value, it is determined that the power remaining in the battery is inadequate. Even if the battery-voltage check circuit determines that sufficient power is left in the battery since the load is rather light, the out- put voltage of the battery may drops below the reference value, due to the internal resistance of the battery when a mechanical unit is driven and, hence, the load increases. Consequently, the mechanical unit can no longer operate normally. The probability is high that such a voltage drop will cause a FDD to make errors in writing data.
FIG. 9 represents the discharge characteristic of a dry cell. Generally, the reference voltage set in the battery-voltage check circuit is slightly higher than the minimum voltage which can correctly drive the load provided within the battery-driven apparatus. This is because a back-up operation, such as writing data on a disk inserted in the FDD, must be performed as soon as the check circuit detects an insufficiency of power. At point A on the light-load characteristic line (FIG. 9), the power remaining in the dry cell is slightly greater than the reference value. When the load suddenly increases, the check circuit detects an insufficiency of power, as is indicated by point B on the heavy-load characteristic line (FIG. 9). Since point B is far below the horizontal line representing the reference value of the output voltage of the dry cell, the load can no longer operate normally.
FIG. 10 shows the discharge characteristic of a Ni-Cd battery (secondary battery). When the Ni-Cd battery is used, the same problem arises as in the case of using the dry cell having the discharge characteristic shown in FIG. 9. In addition, the battery-voltage check circuit detects an insufficiency of power before the Ni-Cd battery is used up. Since the user replaces the battery with a new one when the check circuit detects the power insufficiency, the power left in the battery will be wasted.